staging: lustre: delete the filesystem from the tree.

The Lustre filesystem has been in the kernel tree for over 5 years now.
While it has been an endless source of enjoyment for new kernel
developers learning how to do basic codingstyle cleanups, as well as an
semi-entertaining source of bewilderment from the vfs developers any
time they have looked into the codebase to try to figure out how to port
their latest api changes to this filesystem, it has not really moved
forward into the "this is in shape to get out of staging" despite many
half-completed attempts.

And getting code out of staging is the main goal of that portion of the
kernel tree.  Code should not stagnate and it feels like having this
code in staging is only causing the development cycle of the filesystem
to take longer than it should.  There is a whole separate out-of-tree
copy of this codebase where the developers work on it, and then random
changes are thrown over the wall at staging at some later point in time.
This dual-tree developm...